CANNABIS may be a class B drug, which carries a maximum five-year prison sentence for possession, but publicly, at least, it is regarded in far less sinister terms than its other illicit cousins.

Some experts, including Professor David Nutt, the government’s former chief drug advisor, have described it as relatively harmless.

But there are also concerns the availability of stronger forms of the drug – much more potent than the pot available in the 1960s and 1970s – has led many to claim a new form of illness, cannabis-induced psychosis, has emerged as a result.

Statistics from the Royal College of Psychiatrists suggest two million people in the UK smoke cannabis, while half of 16 to 29-year-olds have tried the drug at least once.

But many of its users will be blissfully unaware it can cause a range of unpleasant side effects, including paranoia, anxiety, nausea, infertility, and, in extreme cases, mental illness, in the form of psychosis.

Psychosis is a common symptom of conditions such as schizophrenia and bipolar disorder and a feature of drug and alcohol abuse.

It is estimated that one in every 200 people in the UK have experienced an episode of psychosis at some point in their lives, although not all of these incidents will be drug-related.

Symptoms include hallucinations, delusions of grandeur, extreme paranoia, confusion of thought, the inability to recognise strange behaviour and a refusal to accept reality.

Dr Stanley Zammit, a clinical senior lecturer at Cardiff University’s School of Medicine, said: “It is impossible to say whether psychosis has been caused by the use of cannabis in individual cases, but there is clearly a link between the two as previous sufferers have relapsed as a result of taking cannabis.

“There’s a 1% chance that non cannabis users will suffer from schizophrenia at some point in their lives but there’s strong evidence to suggest that regular users of cannabis are twice as likely to become schizophrenic.

“There is also a greater risk of contracting lesser illnesses.”

But Dr Zammit highlighted the limited evidence available about the psychotic effects of cannabis, saying: “It’s hard to tell whether cannabis is a direct cause of psychosis or if it serves as a catalyst to underlying problems.”

Ian Hindmarch, emeritus professor of human psychopharmacology, said: “It is very rare to find a psychotic state of mind in a mild user.”

But he said he believed there was an increased risk of experiencing psychosis with the regular use of stronger forms of cannabis, such as hashish.

“As a result of repeatedly taking this stronger substance, users can develop the symptoms of a full-blown psychotic,” he said.

“This form of psychosis has become more prevalent where sufferers have become completely detached from reality.”

Prof Hindmarch said the physical effects of cannabis were secondary compared with its effect upon judgement.

“Cannabis distorts the sense of the passage of time which can affect general performance and personal judgement. The biggest risk is to smoke cannabis and drive a car.”

Janet Roberts, manager of the Wales Drug and Alcohol Network, said the relationship between cannabis and psychosis is a two-way street.

“In many cases, sufferers of schizophrenia will self-medicate with cannabis in order to control their symptoms, but this inevitably makes matters worse.”

Asked whether people are aware about theses effects of using cannabis, she added: “There is information to suggest that people are aware, from looking on websites such as Talk to Frank.

“The problem lies in that a lot of people have known somebody who has taken drugs and been fine. The issue is that the way our bodies react to drugs varies from person to person.”

Ms Roberts said the biggest problem facing people who need help is delusion.

“A lot of people who we come into contact with believe they don’t have a problem. Sometimes they have to hit rock-bottom before they can accept that they need help.”